1. Field of the Invention
The present invention relates to an improvement of a differential gear having a differential restricting mechanism.
2. Description of the Prior Art
Recently, a viscous clutch such as the one disclosed in the Japanese Laid-open Utility Model No. 47-203 tends to be used instead of a friction clutch for the differential restricting mechanism.
The viscous clutch utilizes the viscous resistance of a viscous fluid to realize a differential restricting action according to differential rotation between resistance plates which are provided for, for instance, axles and a differential casing respectively.
Comparing the viscous clutch with the frictional clutch, it will be seen that the frictional clutch does not cause a rise in torque even when a differential speed is increased so that the restricting effect thereof may not vary. On the other hand, the viscous clutch can realize the restricting effect in response to the differential speed, and it causes no noise, less wear, and a smooth action, because the viscous clutch is a non-contact type clutch.
When the viscous clutch is subjected to a differential action, however, temperature as well as pressure in an operation chamber of the viscous clutch will be increased, increasing a chance of breaking seals and deteriorating functions.
The viscous clutch is constituted such that the differential restricting effect thereof is effected only when a differential rotation is caused. Therefore, in a circumstance that wheels get in the mud, the transmission of torque through the viscous clutch is delayed momentarily so that it may not be quick to get out of the mud compared to the friction clutch.
Another conventional differential gear is disclosed in Japanese Laid-Open Patent 51-99769 in which a differential gear is provided with a mechanism for restricting the differential operation using a viscous coupling. The viscous coupling has a structure in which a plurality of resistance plates opposite each other are disposed on the axle side and on the differential casing side for example within an operation chamber filled with a viscous fluid.
When a rotational difference between the resistance plates is generated due to slip, etc. of a wheel, the resistance plates are rotated while shearing the viscous fluid so that a great shearing resistance is applied to the viscous coupling and restricts the differential operation.
However, the viscous coupling utilizes the viscous resistance of the viscous fluid at the shearing time when the viscous fluid is sheared by the difference of the number of rotations between the resistance plates. Accordingly, when the viscous fluid is sheared, i.e., the differential operation is continued for a long time, the temperature of the viscous fluid is increased and thereby the pressure within the operation chamber is increased so that it is possible to damage a seal member, etc. of the viscous coupling.
To overcome these problems, it is considered to use a means for preventing the continuous differential operation by actuating a friction clutch mechanism by the pressure within the viscous coupling when the pressure within the viscous coupling is increased. Such a preventing means is shown in FIG. 1. In FIG. 1, a mechanical friction clutch mechanism 305 is located outside a viscous coupling 307 within a differential casing 301 and is disposed between a rear portion of one side gear 303 and the differential casing 301. In such a structure, when the pressure within the viscous coupling 307 is increased, the other side gear 311 forming an operation chamber 309 of the viscous coupling 307 is moved in the axial direction and transmits this axial movement to the clutch mechanism 307 through a pinion gear 313 meshing with the side gear 311, and the side gear 303. Thus, the clutch mechanism 307 is coupled, thereby restricting the differential operation.
However, in such a conventional apparatus, when the pressure F within the operation chamber 309 is increased, the pressure F is transmitted to the clutch mechanism 305 through the side gears 311 and 303 meshing with the pinion gear 313 at any time, so that there is no back-lash of the respective gears, thereby sometimes damaging them or no smooth differential operation being obtained.